The light output of a light emitting diode is generally controlled by regulating a current level of a LED current through the LED. The LED current may be further modulated with, e.g. a pulse width modulation (PWM) scheme. In such a PWM-scheme, the LED receives the LED current in a periodic sequences of pulses of a certain width, while the width of the pulses is modulated from a first pulse width to a second pulse width when the effective light output is to be changed from a first light output level to a second light output level.
A LED drive method and a LED drive circuit thus generally comprise a current source, providing a constant current or an oscillating current with an average current level, and a switch associated with the LED in order to control a path of the current and in order to achieve the pulse width modulation of the LED current.
The switch may be in series with the LED, thus controlling the path of the current by interrupting the path of the current in order to achieve the pulse width modulation.
The switch may alternatively be in parallel with the LED, which will be referred to as a bypass switch. The bypass switch controls the path of the current by either guiding the path of the current through the LED or guiding the path of the current through a bypass path parallel to the LED in order to achieve the pulse width modulation. One of the advantages of such a bypass switch approach is that the current continues to flow, either through the LED or though the bypass path, which allows the use of very efficient current sources, such as a switch-mode current source. This is especially advantageous when a plurality of LEDs are to be operated at a common current level but with a possibly different pulse width between different LEDs from the plurality of LEDs. The LEDs may then be arranged in a plurality of LED segments connected in series, each LED segment comprising a single LED or two or more LEDs, the two or more LEDs preferably arranged in series, and each of the LED segments being associated with a bypass switch in parallel to the corresponding LED segment. By operating the bypass switches independently, the effective light output of each of the LED segments may be varied independently.
An example of a current source is described in WO 2004/100614A1. WO 2004/100614A1 describes a LED current control method and circuit for accurately and quickly regulating the mean amperage of LED current during all operating conditions including a change in the input line of a power source or in a change in a load of the LED network.
The method comprises controlling the LED current to oscillate, e.g. in a triangular or saw-tooth manner, between a peak amperage and a valley amperage, with the mean amperage being the average of the peak amperage and the valley amperage, by an alternate controlling of an increase and a decrease of the LED current in response to each crossover by a converter current sensing voltage of a lower trip voltage and an upper trip voltage in a negative and a positive direction respectively. A circuit using such a method may be referred to as an example of a switch-mode converter with hysteretic control on the LED current. The peak-to-valley range of the peak amperage to the valley amperage may be referred to as the hysteretic current window. The peak-to-valley range of the upper trip voltage to the lower trip voltage may be referred to as the hysteretic voltage window, or, in short, the hysteretic window.
The method and circuit thus achieve regulating the mean current level independent of the operating conditions. In particular, when the method and circuit are used to operate a LED circuit arrangement comprising a plurality of LED segments with corresponding bypass switches in an arrangement as described above. Operating the bypass switches to vary the light output of the individual LED segments results in a variation of the load of the LED circuit arrangement. The switch-mode converter with hysteretic control is well suited to accurately and quickly deliver a current with a substantially constant mean current level to such a LED circuit arrangement with varying load due to the operation of the bypass switches.
However, when increasing the switching frequency in a switch-mode converter like the hysteretic converter, the controlling of the increase and decrease of the current in response to a crossover of the converter current sensing voltage of the upper of lower trip voltage will not be perfect. The change of increasing to decreasing, or vice versa, may not be immediate, e.g. due to circuit delays, and may be associated with overshoots above the peak amperage and undershoots below the valley amperage. These over- and undershoots may e.g. find a cause in a relative increase of the effect of parasitic capacitance. Thus, although the oscillation is controlled to be within substantially the peak amperage and the valley amperage, this control is not perfect and inaccuracies occur. As a result, the achieved mean current level may deviated from the intended mean current level.
Hence, it is a problem of the known switch-mode converters that increasing the frequency is associated with an increased inaccuracy of the achieved mean current level. This hampers the increase of the frequency which is on the other hand preferred e.g. in order to reduce the total cost of the circuit, to reduce the size of inductors or capacitors, to reduce the required space of a LED driver circuit in a LED lighting system, to allow full integration of inductors and/or capacitors in a LED driver IC, and/or to obtain an improved response time, e.g. when dimming, e.g. due to a faster capacitive discharging.